To add what Rudy Stricklan states, the nominal township is a square, six miles on a side, containing 36 evenly-divided sections. However, townships and their subdivisions may deviate - both in number and in shape - for a variety of reasons, from the difficulties encountered while surveying through rugged terrain to fraudulent 1800's era survey reports compiled not in the field, but at the local watering hole (or so I've been told - I'm not a surveyor).
For example, in Washington State, half-townships were added to "fill in the gaps" between different initial survey projects that did not properly meet up. Additionally, there are other types of subdivisions in addition to sections (including donation land claims, federal tracts, and protracted blocks). To see this complexity first-hand, Washington state townships/section data can be downloaded here, and here's the metadata.
Perhaps your area of interest will not include any of the anomalies I've mentioned. On the other hand, you may want to account for all potentialities.